[Animats]

Agree on title. Too dramatic.

The author seems to be obsessing about the overhead for trivial functions. He's bothered by overhead for states for "panicked" and "returned". That's not a big problem. Most useful async blocks are big enough that the overhead for the error cases disappears.

He may have a point about lack of inlining. But what tends to limit capacity for large numbers of activities is the state space required per activity.

[molf]

> Most useful async blocks are big enough that the overhead for the error cases disappears.

Is it really though?

In my experience many Rust applications/libraries can be quite heavy on the indirection. One of the points from the article is that contrary to sync Rust, in async Rust each indirection has a runtime cost. Example from the article:

    async fn bar(blah: SomeType) -> OtherType {
       foo(blah).await
    }

I would naively expect the above to be a 'free' indirection, paying only a compile-time cost for the compiler to inline the code. But after reading the article I understand this is not true, and it has a runtime cost as well.

[selfhoster1312]

In my experience, it's not uncommon to have an async trait method for which many implementations are actually synchronous. For example, different tables in your DB need to perform some calculations, but only some tables reference other tables. In that case, the method needs to be async and take a handle to the DB as parameter, but many table entries can perform the calculation on their own without using the handle (or any async operation).

This may look like a case of over-optimization, but given how many times i've seen this pattern, i assume it builds up to a lot of unnecessary fluff in huge codebases. To be clear, in that case, the concern is not really about runtime speed (which is super fast), but rather about code bloat for compilation time and binary size.

[swiftcoder]

> Most useful async blocks are big enough that the overhead for the error cases disappears.

Most useful async blocks are deeply nested, so the overhead compounds rapidly. Check the size of futures in a decently large Tokio codebase sometime

[algesten]

He's optimizing for embedded no-std situation. These things do matter in constrained environments.

[Animats]

He briefly mentions microcontrollers, but doesn't go into details. That might make sense, for microcontrollers which implement some kind of protocol talking to something.

[repelsteeltje]

> [...] That's not a big problem [...]

Depends somewhat on your expectations, I suppose. Compared to Python, Java, sure, but Rust off course strives to offer "zero-cost" high level concepts.

I think the critique is in the same realm of C++'s std::function. Convenience, sure, but far from zero-cost.

[pjmlp]

To the point it got replaced by std::function_ref() in C++26.

[repelsteeltje]

Exactly. And I guess that is also the gist of the article: async Rust needs additional TLC.

[dathinab]

> Agree on title. Too dramatic.

not just too dramatic

given that all the things they list are

non essential optimizations,

and some fall under "micro optimizations I wouldn't be sure rust even wants",

and given how far the current async is away from it's old MVP state,

it's more like outright dishonest then overly dramatic

like the kind of click bait which is saying the author does cares neither about respecting the reader nor cares about honest communication, which for someone wanting to do open source contributions is kinda ... not so clever

through in general I agree rust should have more HIR/MIR optimizations, at least in release mode. E.g. its very common that a async function is not pub and in all places directly awaited (or other wise can be proven to only be called once), in that case neither `Returned` nor `Panicked` is needed, as it can't be called again after either. Similar `Unresumed` is not needed either as you can directly call the code up to the first await (and with such a transform their points about "inlining" and "asyncfns without await still having a state machine" would also "just go away"TM, at least in some places.). Similar the whole `.map_or(a,b)` family of functions is IMHO a anti-pattern, introducing more function with unclear operator ordering and removal of the signaling `unwrap_` and no benefits outside of minimal shortening a `.map(b).unwrap_or(a)` and some micro opt. is ... not productive on a already complicated language. Instead guaranteed optimizations for the kind of patterns a `.map(b).unwrap_or(a)` inline to would be much better.